Without a show record in the H/J world, it would be hard to get six figures IMO. The choice may be between selling him for a bit less now vs. investing money in getting him a show record and potentially selling him for more later. But think carefully about how much it will cost to get that record (training fees, board, vet expenses, show fees, risk of injury, etc,). There’s always some degree of risk in waiting it out, but there could be a reward too.
How old is he? How tall is he? Why didn’t he make a successful upper level eventer? Does he have the scope to jump around 1.40/1.45 classes competitively? Is he amateur friendly? Is he fancy/versatile enough to show in all 3 rings? How much do you think he would be worth as an eventer? These questions are all going to be important in terms of pricing and marketing.
I’m an eventer turned show-jumper who is currently doing dressage. I’ve found that philosophies about things like turnout, competition schedules, conditioning, etc. vary as much within disciplines as across them. A lot of this will also have to do with location. In Northern CA, for example, everyone wants generous turnout but it’s hard to come by since land is so expensive. It’s hard to generalize. Your best bet would be to look at specific barns when people are trying him. Do they go to every show on the calendar? Do the horses all jump multiple classes on multiple days? Does their facility have decent turnout?
PS - Similar to @Renn/aissance, I love the Irish eventers. I’ll be in the market for a jumper prospect myself in the next year and will be targeting “flunked” event horses, partially for budgetary reasons and partially because it’s the type of ride I’m accustomed to. However, many riders might not want to take the risk (at least for a high 5s price tag) on something without an established record.
Best of luck!